Sulfide solid electrolyte and all solid state battery

ABSTRACT

A sulfide solid electrolyte is capable of suppressing a decrease in Li ion conductivity due to moisture. A sulfide solid electrolyte includes a Li element, a P element, a S element and an O element, and having a granular shape, and including a crystal portion oriented along the granular shape, on an inner surface of the sulfide solid electrolyte.

TECHNICAL FIELD

The present disclosure relates to a sulfide solid electrolyte capable ofsuppressing a decrease in Li ion conductivity due to moisture.

BACKGROUND ART

An all solid state battery is a battery including a solid electrolytelayer between a cathode active material layer and an anode activematerial layer, and having an advantage that, compared to a liquidbattery including a liquid electrolyte containing a flammable organicsolvent, it is easier to simplify the safeguard thereof.

As a solid electrolyte used for the all solid state battery, a sulfidesolid electrolyte has been known. Patent Literature 1 discloses, forexample, a method for producing a crystallized sulfide solid electrolytematerial comprising: a step of amorphizing a raw material compositionincluding Li₂S and P₂S₅ in a predetermined proportion, and then, a stepof heat treating under specific conditions. The challenge of thistechnique is to reduce the generating amount of hydrogen sulfide.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-Open (JP-A)    No. 2010-218827

SUMMARY OF INVENTION Technical Problem

The Li ion conductivity of the sulfide solid electrolyte tends to bedecreased due to moisture (such as moisture in an atmosphere). Thepresent disclosure has been made in view of the above circumstances, anda main object thereof is to provide a sulfide solid electrolyte capableof suppressing a decrease in Li ion conductivity due to moisture.

Solution to Problem

The present disclosure provides a sulfide solid electrolyte comprising aLi element, a P element, a S element and an O element, and having agranular shape, and including a crystal portion oriented along thegranular shape, on an inner surface of the sulfide solid electrolyte.

According to the present disclosure, by including a crystal portion onan inner surface of the sulfide solid electrolyte, a sulfide solidelectrolyte capable of suppressing a decrease in Li ion conductivity dueto moisture may be obtained.

In the disclosure, the sulfide solid electrolyte may have a compositionrepresented by Li_(3+x)PS_(4−y)O_(y), wherein x satisfies 0≤x≤1, and ysatisfies 0<y<4.

In the disclosure, the x may satisfy 0≤x≤0.2, and the y may satisfy0.8≤y≤1.2.

In the disclosure, a thickness of the crystal portion may be 1.7 nm ormore.

The present disclosure also provides an all solid state batterycomprising a cathode active material layer including a cathode activematerial, an anode active material layer including an anode activematerial, and a solid electrolyte layer formed between the cathodeactive material layer and the anode active material layer, and at leastone of the cathode active material layer, the anode active materiallayer, and the solid electrolyte layer includes the above describedsulfide solid electrolyte.

According to the present disclosure, by using the above describedsulfide solid electrolyte, the output property of an all solid statebattery may be maintained even, for example, under a high humidityenvironment.

Advantageous Effects of Invention

The sulfide solid electrolyte in the present disclosure exhibits aneffect that a decrease in Li ion conductivity due to moisture may besuppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are schematic diagrams explaining the sulfide solidelectrolyte in the present disclosure.

FIG. 2 is a schematic cross-sectional view showing an example of the allsolid state battery in the present disclosure.

FIG. 3 is a TEM image of the sulfide solid electrolyte obtained inExample 1.

FIG. 4 is a TEM image of the sulfide solid electrolyte obtained inComparative Example 1.

FIG. 5 is the result of the XRD measurement to the sulfide solidelectrolyte obtained in Example 1 and Comparative Example 1.

FIG. 6 is the result of the NMR measurement to the sulfide solidelectrolyte obtained in Example 1 and Comparative Example 1.

DESCRIPTION OF EMBODIMENTS

A sulfide solid electrolyte and an all solid state battery in thepresent disclosure are hereinafter described in detail.

A. Sulfide Solid Electrolyte

FIGS. 1A to 1C are schematic diagrams explaining the sulfide solidelectrolyte in the present disclosure. FIG. 1A is a schematiccross-sectional view showing an example of the sulfide solidelectrolyte. Sulfide solid electrolyte 10 shown in FIG. 1A comprises aLi element, a P element, a S element and an O element, and has agranular shape. Also, FIG. 1B is an enlarged view wherein the vicinityof the surface of the sulfide solid electrolyte in FIG. 1A is enlarged.In the present disclosure, as shown in FIG. 1B, sulfide solidelectrolyte 10 includes crystal portion 1 oriented along the granularshape, on an inner surface. As shown in FIG. 1B, “inner surface” refersto the surface on sulfide solid electrolyte 10 side, with respect tointerface I of the particle. Incidentally, as described in the laterdescribed Comparative Examples, when an amorphizing treatment and a heattreatment are carried out, amorphous portion 2 tends to be formed on theinner surface of sulfide solid electrolyte 10 as shown in FIG. 1C.

According to the present disclosure, by including a crystal portion onan inner surface of the sulfide solid electrolyte, a sulfide solidelectrolyte capable of suppressing a decrease in Li ion conductivity dueto moisture may be obtained. Here, Patent Literature 1 discloses, amethod for producing a crystallized sulfide solid electrolyte materialcomprising: a step of amorphizing a raw material composition includingLi₂S and P₂S₅ in a predetermined proportion, and then, a step of heattreating under specific conditions. In Patent Literature 1, thegeneration of the hydrogen sulfide due to moisture is suppressed byadjusting the proportion of Li₂S and P₂S₅ to include more PS₄ ³⁻structure with good water resistance.

Meanwhile, in Patent Literature 1, although the generation of thehydrogen sulfide due to moisture may be suppressed, there is a newproblem that Li ion conductivity is remarkably decreased when thesulfide solid electrolyte becomes into contact with moisture. Theaccumulation of utmost research in view of the above circumstances haveled the present inventors to find out that the remarkable decrease of Liion conductivity is likely to be caused by the nanometer order sizedamorphous portion existing on the inner surface of the sulfide solidelectrolyte. Therefore, the present inventors formed a crystal portion,not the amorphous portion, on the inner surface of the sulfide solidelectrolyte, and it was confirmed that a decrease in Li ion conductivitydue to moisture may be suppressed. Incidentally, since measuring methodsto investigate the condition of the nanometer order sized region, areextremely limited, it is difficult to predict the phenomenon that occursin the nanometer order sized region. However, the present inventors havestudied the phenomenon carefully, and have solved the new problem thatLi ion conductivity is remarkably decreased.

The sulfide solid electrolyte in the present disclosure comprises a Lielement, a P element, an S element and an O element. The proportion ofthe Li element, the P element, the S element and the O element to allelements contained in the sulfide solid electrolyte is, for example, 70mol % or more, may be 80 mol % or more, and may be 90 mol % or more.Further, the proportion of the O element to the sum of the S element andthe O element is, for example, 5 mol % or more, may be 10 mol % or more,and may be 20 mol % or more. On the other hand, the above describedproportion of the O element is, for example, 70 mol % or less, may be 60mol % or less, and may be 50 mol % or less.

The sulfide solid electrolyte has a granular shape. The average particlesize (D₅₀) of the sulfide solid electrolyte is, for example, 0.1 μm ormore, may be 0.5 μm or more, and may be 1 μm or more. Meanwhile, theaverage particle size (D₅₀) of the sulfide solid electrolyte is, forexample, 50 μm or less, and may be 30 μm or less. The average particlesize (D₅₀) may be determined from, for example, the results of particlesize distribution measured by a laser diffraction scattering method.

The sulfide solid electrolyte includes a crystal portion oriented alongthe granular shape on an inner surface. By providing the crystalportion, a decrease in Li ion conductivity due to moisture may besuppressed. Also, the crystal portion is oriented along the granularshape. In other words, the orientation of the crystal plane in thecrystal portion is along the granular shape (strictly, the outerperipheral shape of the particle). Such orientation occurs, for example,when a melt quenching method is used. That is, such orientation occursin the process of decreasing the temperature from the outside toward theinside during quenching.

Among the all surface regions of the sulfide solid electrolyte, theproportion of the region where the crystal portion is formed is, forexample, 90% or more, may be 95% or more, and may be 99% or more. Thisproportion may be determined, for example, by observation with anelectron microscope. Further, the thickness of the crystal portion is,for example, 1.7 nm or more, may be 5 nm or more, and may be 10 nm ormore. Meanwhile, the thickness of the crystal portion is, for example,200 nm or less, and may be 100 nm or less. Also, as described in thelater described Examples, the thickness of the crystal portion may notbe even in some cases. In view of this point, the proportion of theregion wherein the crystal portion with thickness of 1.7 nm or more isformed, among the all surface regions of the sulfide solid electrolyteis, for example, 90% or more, may be 95% or more, and may be 99% ormore. In this case, the preferable thickness of the crystal portion isthe same as that described above.

The sulfide solid electrolyte usually includes the crystal portion and amidportion located at inner side than the crystal portion. The crystalportion and the midportion have continuity as a material. In otherwords, there is no distinct interface between the crystal portion andthe midportion, and the both are elements of a single material. Thecrystal portion contains a Li element, a P element, an S element and anO element. Further, the crystal portion has crystallinity. Inparticular, it is preferable that the crystal portion has the laterdescribed crystal phase. On the other hand, the midportion also containsa Li element, a P element, an S element and an O element. Further, it ispreferable that the midportion also has crystallinity. In particular,the midportion preferably has the same crystal phase as the crystalportion. Also, the crystal portion and the midportion may have the samecomposition and may have different compositions. Specific examples ofthe latter may include a case where the crystal portion is segregated.

The sulfide solid electrolyte preferably includes an anion structurerepresented by PS_(4−α)O_(α) ³⁻ (a is an integer of 0 or more and 4 orless) as a main component of the anion structure. The anion structurerepresented by a PS_(4−α)O_(α) ³⁻ includes a PS₄ ³⁻, PS₃O³⁻, PS₂O₂ ³⁻,PSO₃ ³⁻, and PO₄ ³⁻. Among them, it is preferable that the sulfide solidelectrolyte in the present disclosure includes PS₄ ³⁻ as a maincomponent of the anion structure. The reason therefore is to obtain highLi ion conductivity. The proportion of PS₄ ³⁻ to all the anionstructures in the sulfide solid electrolyte is preferably, for example,50 mol % or more.

The sulfide solid electrolyte preferably includes no Li₂S. The reasontherefore is to suppress the generation of hydrogen sulfide. When Li₂Sis used as the starting material, for example, it is preferable that theLi₂S is not remained. “Including no Li₂S” may be confirmed by X-raydiffraction (XRD). Specifically, it is preferable that no Li₂S peaks(2θ=27.0°, 31.2°, 44.8°, and 53.1°) are observed in the XRD-measurementusing CuKα ray.

It is preferable that the sulfide solid electrolyte includes nocrosslinked sulfur. The reason therefore is to suppress the generationof hydrogen sulfide. Specifically, the crosslinked sulfur is an anionstructure represented by P₂S₇ ⁴⁻. “Including no crosslinked sulfur” maybe confirmed by Raman spectroscopy. Specifically, the peak of P₂S₇ ⁴⁻appears in the vicinity of 402 cm⁻¹, and the peak of PS₄ ³⁻ appears inthe vicinity 417 cm⁻¹. In the present disclosure, it is preferable thatintensity I₄₀₂ of the 402 cm⁻¹ is lower than intensity I₄₁₇ of the 417cm⁻¹. With respect to the intensity I₄₁₇, the intensity I₄₀₂ is, forexample, 70% or less, may be 50% or less, and may be 35% or less. Inparticular, it is preferable that the peak of P₂S₇ ⁴ is not observed.

The composition of the sulfide solid electrolyte is not particularlylimited. The sulfide solid electrolyte preferably has a compositionrepresented by, for example, Li_(3+x)PS_(4−y)O_(y) (x satisfies 0≤x≤1, ysatisfies 0<y<4). In the above composition, x may be 0, and may begreater than 0. On the other hand, x is usually 1 or less, may be 0.5 orless, and may be 0.2 or less. Also, in the above composition, y may be0.5 or more, and may be 0.8 or more. On the other hand, y may be 2 orless, may be 1.5 or less, and may be 1.2 or less.

It is preferable that the sulfide solid electrolyte includes a crystalphase of a LGPS type. The reason therefore is to improve the Li ionconductivity. This crystal phase is regarded as crystal phase A. Thecrystal phase A has a peak at positions of 2θ=13.0°±0.5°, 15.4°±0.5°,18.0°±0.5°, 21.1°±0.5°, 21.8°±0.5°, 24.3°±0.5°, 25.5°±0.5°, 28.4°±0.5°,30.9°±0.5°, and 33.9°±0.5° in X-ray diffraction measurement using CuKαray. Incidentally, these peak positions are defined in a range of ±0.5°because they may be varied according to, for example, the composition ofthe sulfide solid electrolyte. Incidentally the position of each peakmay be in a range of ±0.3°, and may be in a range of ±0.1°. Also, it ispreferable that the sulfide solid electrolyte in the present disclosureincludes crystal phase A as a main phase. “Including as a main phase”means that the proportion of the crystal phase is the highest among allthe crystal phases included in the sulfide solid electrolyte. Theproportion of the crystal phase is, for example, 50 weight % or more,may be 70 weight % or more, and may be 90 weight % or more. Incidentallythe proportion of the crystal phase may be measured by, for example,radiation XRD.

It is preferable that the sulfide solid electrolyte has high Li ionconductivity. The Li ion conductivity of the sulfide solid electrolyteat 25° C. is, for example, 1×10⁻⁵ S/cm or more, and preferably 1×10⁻⁴S/cm or more.

Since the sulfide solid electrolyte in the present disclosure has goodLi ion conductivity, it may be used in any application requiring Li ionconductivity. Among them, the sulfide solid electrolyte in the presentdisclosure is preferably used for an all solid state battery. Forexample, even in a high humidity environment, an all solid state batterywith good output property may be obtained.

The method for producing the sulfide solid electrolyte in the presentdisclosure is not particularly limited. Examples of the method forproducing a sulfide solid electrolyte may include a method including apreparation step of preparing a raw material composition containing aconstituent component of the sulfide solid electrolyte, and a meltquenching step of heating, melting, and quenching the raw materialcomposition. Incidentally, in the present disclosure, by appropriatelyadjusting the conditions of melt quenching, a sulfide solid electrolyteincluding a desired crystal portion may obtained.

The raw material composition contains a Li element, a P element, an Selement and an O element. Examples of the raw material containing Lielement may include a sulfide of Li. Examples of the sulfide of Li mayinclude Li₂S. Examples of the raw material containing the P element mayinclude a simple substance of P and a sulfide of P. Examples of thesulfide of P may include P₂S₅. Examples of the raw material containingthe S element may include a simple substance of S, a sulfide of Li, anda sulfide of P. Examples of the raw material containing an O element mayinclude an oxide of Li and an oxide of P. Examples of the oxide of Limay include Li₂O. Examples of the oxide of P may include P₂O₅.

The raw material composition may be obtained, for example, by mixingeach raw material. It is preferable that the proportion of each rawmaterial is appropriately adjusted in consideration of the compositionof the target sulfide solid electrolyte. Although there is no particularlimitation on the method of mixing the raw materials, for example, amethod of mixing the raw materials while pulverizing them is preferred.This is because a more uniform raw material composition may be obtained.Examples of a method of mixing the raw materials while pulverizing mayinclude a vibration mill.

The heating temperature when heating the raw material composition is,for example, 700° C. or more, may be 800° C. or more, and may be 900° C.or more. On the other hand, the heating temperature is, for example,1200° C. or less, and may be 1100° C. or less. Further, the heating timeis, for example, 30 minutes or more, and may be 1 hour or more. On theother hand, the heating time is, for example, 100 hours or less, and maybe 50 hours or less. Also, it is preferable that the heating atmosphereis under a vacuum or an inert gas atmosphere from the viewpoint ofpreventing oxidation. Examples of the heating method may include amethod using a firing furnace.

By quenching the melted raw material composition, a sulfide solidelectrolyte including a desired crystal portion may be obtained. Thecooling rate is, for example, 500° C./minute or more, and may be 700°C./minute or more. Further, it is preferable to cool by quenching to,for example, 100° C. or less, more preferably to 50° C. or less. As thecooling method, a method wherein the melted raw material composition isdirectly or indirectly contacted with a refrigerant, is usually used.Specific examples thereof may include a method of bringing a containercontaining a melted raw material composition into contact with a liquidsuch as water and ice water, and a method of bringing the melted rawmaterial composition into contact with a rotating metal roll.

B. All Solid State Battery

FIG. 2 is a schematic cross-sectional view showing an example of the allsolid state battery in the present disclosure. All solid state battery100 shown in FIG. 2 comprises cathode active material layer 11 includinga cathode active material, anode active material layer 12 including ananode active material, solid electrolyte layer 13 formed between cathodeactive material layer 11 and anode active material layer 12, cathodecurrent collector 14 for collecting current of cathode active materiallayer 11, anode current collector 15 for collecting current of anodeactive material layer 12, and a battery case 16 for storing thesemembers. A characteristic in the present disclosure is that, at leastone of cathode active material layer 11, anode active material layer 12,and electrolyte layer 13 includes the above described sulfide solidelectrolyte.

According to the present disclosure, by using the above describedsulfide solid electrolyte, the output property of an all solid statebattery may be maintained even, for example, under a high humidityenvironment.

1. Cathode Active Material Layer

The cathode active material layer is a layer including at least acathode active material, and may include at least one of a solidelectrolyte, a conductive material, and a binder, if necessary. Inparticular, in the present disclosure, it is preferable that the cathodeactive material layer includes the above described sulfide solidelectrolyte. The proportion of the sulfide solid electrolyte included inthe cathode active material layer is, for example, 0.1 volume % or more,may be 1 volume % or more, and may be 10 volume % or more. On the otherhand, the proportion of the sulfide solid electrolyte included in thecathode active material layer is, for example, 80 volume % or less, maybe 60 volume % or less, and may be 50 volume % or less. Further,examples of the cathode active material may include oxide activematerials such as LiCoO₂, LiMnO₂, Li₂NiMn₃O₈, LiVO₂, LiCrO₂, LiFePO₄,LiCoPO₄, LiNiO₂, LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂.

The cathode active material layer may include a conductive material. Byadding the conductive material, the conductivity of the cathode activematerial layer may be improved. Examples of the conductive material mayinclude carbon materials such as acetylene black, Ketjen black, andcarbon fiber. Further, the cathode active material layer may include abinder. Examples of the binder may include a fluorine based binder suchas polyvinylidene fluoride (PVDF). Further, the thickness of the cathodeactive material layer is, for example, 0.1 μm or more and 1000 μm orless.

2. Anode Active Material Layer

The anode active material layer is a layer including at least an anodeactive material, and may include at least one of a solid electrolyte, aconductive material, and a binder, if necessary. In particular, in thepresent disclosure, it is preferable that the anode active materiallayer includes the above described sulfide solid electrolyte. Theproportion of the sulfide solid electrolyte included in the anode activematerial layer is, for example, 0.1 volume % or more, may be 1 volume %or more, and may be 10 volume % or more. On the other hand, theproportion of the sulfide solid electrolyte included in the anode activematerial layer is, for example, 80 volume % or less, may be 60 volume %or less, and may be 50 volume % or less. Further, examples of the anodeactive material may include a metal active material and a carbon activematerial. Examples of the metal active material may include In, Al, Si,and Sn. On the other hand, examples of the carbon active material mayinclude mesocarbon microbeads (MCMB), highly oriented pyrolytic graphite(HOPG), hard carbon, and soft carbon.

Incidentally, the conductive material and the binder used for the anodeactive material layer are the same as those in the case of the cathodeactive material layer described above. Further, the thickness of theanode active material layer is, for example, 0.1 μm or more and 1000 μmor less.

3. Solid Electrolyte Layer

The solid electrolyte layer is a layer formed between the cathode activematerial layer and the anode active material layer. Further, the solidelectrolyte layer is a layer including at least a solid electrolyte, andmay include a binder if necessary. In particular, in the presentdisclosure, it is preferable that the solid electrolyte layer includesthe sulfide solid electrolyte described above. The proportion of thesulfide solid electrolyte included in the solid electrolyte layer is,for example, 50 volume % or more, may be 70 volume % or more, and may be90 volume % or more. Incidentally, the binder used for the solidelectrolyte layer is the same as in the case of the cathode activematerial layer described above. Further, the thickness of the solidelectrolyte layer is, for example, 0.1 μm or more and 1000 μm or less.

4. Other Configurations

The all solid state battery in the present disclosure comprises at leastthe cathode active material layer, the solid electrolyte layer, and theanode active material layer described above. Further, a cathode currentcollector for collecting current of the cathode active material layerand an anode current collector for collecting current of the anodeactive material layer are usually provided. Examples of the material forthe cathode current collector may include SUS, aluminum, nickel, iron,titanium, and carbon. On the other hand, examples of the material forthe anode current collector may include SUS, copper, nickel, and carbon.

5. All Solid State Battery

The all solid state battery in the present disclosure is preferably anall solid state lithium ion battery. The all solid state battery may bea primary battery, and may be a secondary battery. Among the above, thelatter is preferable, so as to be repeatedly charged and discharged, andis useful as, for example, a car-mounted battery.

Incidentally, the present disclosure is not limited to the embodiments.The embodiments are exemplification, and any other variations areintended to be included in the technical scope of the present disclosureif they have substantially the same constitution as the technical ideadescribed in the claim of the present disclosure and offer similaroperation and effect thereto.

EXAMPLES Example 1

Li₂S, P₂S₅ and P₂O₅ were weighed and mixed with a vibrating mill for 30minutes to obtain the composition of Li₃PS₃O. The resulting mixture wasplaced in a carbon crucible and vacuum sealed in a quartz tube percarbon crucible. The pressure of the vacuum-sealed quartz tube wasapproximately 30 Pa. The vacuum-sealed quartz tube was placed in afiring furnace, heated at 950° C. for 2.5 hours, and then, quenched bycharging to ice water. Thus, a sulfide solid electrolyte was obtained.

Example 2

A sulfide solid electrolyte was obtained in the same manner as inExample 1 except that Li₂S, P₂S₅, P₂O₅ and P were weighed so as toobtain the composition of Li_(3.2)PS_(2.8)O_(1.2).

Example 3

A sulfide solid electrolyte was obtained in the same manner as inExample 1 except that Li₂S, P₂S₅ and P₂O₅ were weighed so as to obtainthe composition of Li₃PS_(3.2)O_(0.8).

Comparative Example 1

Li₂S, P₂S₅ and P₂O₅ were weighed and put into zirconium pots so as toobtain the composition of Li₃PS₃O. Further, zirconia balls were alsointroduced, and mechanical alloying was performed by a planetary ballmill under the conditions of a rotational speed of 380 rpm for 40 hours.Thus, a sulfide glass was obtained. The obtained sulfide glass waspress-molded under 20 MPa, and the obtained pellet was placed in aquartz tube, and sealed in a vacuum. The pressure of the vacuum-sealedquartz tube was approximately 30 Pa. The vacuum-sealed quartz tube wasplaced in a firing furnace, heated at 260° C. for 4 hours, and then,naturally cooled. Thus, a sulfide solid electrolyte was obtained.

Comparative Example 2

A sulfide solid electrolyte was obtained in the same manner as inComparative Example 1 except that Li₂S, P₂S₅, P₂O₃ and P were weighed soas to obtain the composition of Li_(3.2)PS_(2.8)O_(1.2).

Comparative Example 3

A sulfide solid electrolyte was obtained in the same manner as inComparative Example 1, except that Li₂S, P₂S₃ and P₂O₃ were weighed soas to obtain the composition of Li₃PS_(3.2)O_(0.8).

[Evaluation]

<TEM Measurement>

The sulfide solid electrolytes obtained in Examples 1 to 3 andComparative Examples 1 to 3 were observed with a transmission electronmicroscope (TEM). The powder of the obtained sulfide solid electrolytewas supported on a carbon porous mesh in an inert atmosphere, andobservation was performed. Typical results are shown in FIGS. 3 and 4.FIG. 3 is a TEM image of the sulfide solid electrolyte obtained inExample 1, and FIG. 4 is a TEM image of the sulfide solid electrolyteobtained in Comparative Example 1.

As shown in FIG. 3, in Example 1, it was confirmed that a crystalportion oriented along the granular shape was formed on the innersurface of the sulfide solid electrolyte. It is presumed that the reasonwhy the crystal portion was formed on the inner surface of the sulfidesolid electrolyte is that, by performing melt quenching based on theconditions of Example 1, ideal nucleation and nucleation growth occurredon the particle surface. Incidentally, although not shown in the figure,the crystal portion was formed on the entire surface of the sulfidesolid electrolyte. On the other hand, the thickness of the crystalportion was not even, and in the region where the crystal portion wasthin, the thickness was approximately 1.8 nm, and in the region wherethe crystal portion was thick, the thickness was approximately 65 nm.Incidentally, the particle size of this sulfide solid electrolyte wasapproximately 2 μm, and the proportion of the thickness of the crystalportion to the particle size was 0.001 or more and 0.04 or less. On theother hand, as shown in FIG. 4, in Comparative Example 1, an amorphousportion was formed on the inner surface of the sulfide solidelectrolyte, and a crystal portion was formed inside the amorphousportion.

<Water Resistance Evaluation>

The sulfide solid electrolytes obtained in Examples 1 to 3 andComparative Examples 1 to 3 were used to measure Li ion conductivity.First, in a glove box of dew point −80° C., 200 mg of the sulfide solidelectrolyte was weighed, put into a cylinder made of macor, and pressedunder pressure of 4 ton/cm². Both ends of the obtained pellet weresandwiched by SUS pins, and a confining pressure was applied to thepellet by bolting so as to obtain an evaluation cell (without exposure).

Next, an evaluation cell (with exposure) was fabricated. First, thesulfide solid electrolyte was allowed to stand for 6 hours in a glovebox controlled to a dewpoint of −30° C. An evaluation cell (withexposure) was obtained in the same manner as described above, exceptthat the obtained sulfide solid electrolyte (with exposure) was used.

Next, for the evaluation cell (without exposure) and the evaluation cell(with exposure), the Li ion conductivity at 25° C. was calculated by theAC impedance method. For measurement, a solartron 1260 was used, with anapplied voltage of 5 mV and a measurement frequency range of 0.01 MHz to1 MHz. Also, the Li ion conductivity of the evaluation cell (withexposure) relative to the Li ion conductivity of the evaluation cell(without exposure) was determined as a retention rate (%). The resultsare shown in Table 1.

TABLE 1 Li ion conductivity (S/cm) Crystal portion Without WithRetention Composition on inner surface exposure exposure rate (%)Example 1 Li₃PS₃O Exist 1.12 × 10⁻⁴ 8.92 × 10⁻⁵ 79 Comp. Ex. 1 Li₃PS₃ONot exist 1.76 × 10⁻⁴ 1.08 × 10⁻⁴ 61 Example 2 Li_(3.2)PS_(2.8)O_(1.2)Exist 6.68 × 10⁻⁵ 4.82 × 10⁻⁵ 72 Comp. Ex. 2 Li_(3.2)PS_(2.8)O_(1.2) Notexist 8.55 × 10⁻⁵ 4.76 × 10⁻⁵ 56 Example 3 Li₃PS_(3.2)O_(0.8) Exist 1.97× 10⁻⁴ 9.06 × 10⁻⁵ 46 Comp. Ex. 3 Li₃PS_(3.2)O_(0.8) Not exist 2.75 ×10⁻⁴ 6.69 × 10⁻⁵ 24

As shown in Table 1, in Examples 1 to 3, the retention rate was higherthan Comparative Examples 1 to 3, respectively. Particularly, inExamples 1 and 2, the retention rate exceeded 70%, and exceptionalresults were obtained. It is presumed that this was because a crystalportion was formed on the inner surface of the sulfide solid electrolyteso that the crystal portion suppressed an interchange reaction betweenLi ions and protons.

<XRD Measurement and NMR Measurement>

The sulfide solid electrolytes obtained in Example 1 and ComparativeExample 1 were subjected to X-ray diffraction (XRD) measurement and NMRmeasurement. XRD measurement was performed under conditions of using aninert atmosphere, and CuKα radiation. Also, the NMR measurement wascarried out by ³¹P-NMR using the MAS (Magic Angle Spinning) method. Theresults are shown in FIGS. 5 and 6.

As shown in FIG. 5, distinguishing peaks were observed at positions of2θ=13.0°, 15.4°, 18.0°, 21.1°, 21.8°, 24.3°, 25.5°, 28.4°, 30.9°, and33.9°, and it was confirmed that the material was a single phasematerial of the crystal phase A. Further, as shown in FIGS. 5 and 6, itwas confirmed that the sulfide solid electrolyte obtained in Example 1had higher crystallinity than the sulfide solid electrolyte obtained inComparative Example 1. Specifically, in FIG. 5, the half band width ofthe main peak located in the vicinity of 2θ=30.9° was 0.28° in Example1, and was 0.92° in Comparative Example 1. Further, in FIG. 6, the halfband width of the main peak located in the vicinity of δ=89.5 ppm was4.4 ppm in Example 1, and was 7.6 ppm in Comparative Example 1.Generally, melt quenching is known as a method for synthesizingamorphous, and heat treatment is known as a method for enhancingcrystallinity. Surprisingly, the crystallinity of the sulfide solidelectrolyte obtained in Example 1 became higher than the crystallinityof the sulfide solid electrolyte obtained in Comparative Example 1.Further, a crystal portion was unexpectedly formed on the inner surfaceof the sulfide solid electrolyte obtained in Example 1. As describedabove, this crystal portion was effective in suppressing a decrease inLi ion conductivity due to moisture.

REFERENCE SIGNS LIST

-   -   1: crystal portion    -   2: amorphous portion    -   10: sulfide solid electrolyte    -   11: cathode active material layer    -   12: anode active material layer    -   13: solid electrolyte layer    -   14: cathode current collector    -   15: anode current collector    -   16: battery case    -   100: all solid state battery

1.-5. (canceled)
 6. A sulfide solid electrolyte comprising a Li element,a P element, a S element and an O element, and having a granular shape,and including a crystal portion oriented along the granular shape, on aninner surface of the sulfide solid electrolyte.
 7. The sulfide solidelectrolyte according to claim 6, wherein the sulfide solid electrolytehas a composition represented by Li_(3+x)PS_(4−y)O_(y), wherein xsatisfies 0≤x≤1, and y satisfies 0<y<4.
 8. The sulfide solid electrolyteaccording to claim 7, wherein the x satisfies 0≤x≤0.2, and the ysatisfies 0.8≤y≤1.2.
 9. The sulfide solid electrolyte according to claim6, wherein a thickness of the crystal portion is 1.7 nm or more.
 10. Anall solid state battery comprising a cathode active material layerincluding a cathode active material, an anode active material layerincluding an anode active material, and a solid electrolyte layer formedbetween the cathode active material layer and the anode active materiallayer, and at least one of the cathode active material layer, the anodeactive material layer, and the solid electrolyte layer includes thesulfide solid electrolyte according to claim 6.